Isolation and characterization of subunits from the predominant form of Dolichos biflorus lectin

The subunits of the two molecular forms (A and B) of the Dolichos biflorus lectin were isolated by ion-exchange chromatography on DEAE-cellulose in 8.0 M urea. Subunits IA and IIA which comprise the predominant molecular form A of the lectin were found to have molecular weights of 27,700 and 27,300, respectively, as determined by sedimentation equilibrium studies in 8.0 M urea. These subunits have similar amino acid compositions and each have alanine at their amino-terminal ends. Comparison of the IA and IIA subunits by immunodiffusion against antisera to the seed extract as well as to subunits IA and IIA showed no antigenic differences between the two subunits. Carboxyl terminal analyses of subunits IA and IIA with carboxypeptidase A produced an essentially simultaneous release of both leucine and valine residues from subunit IA; no detectable amino acids were released from subunit IIA under identical conditions. The data suggest that the molecular form A of the lectin (molecular weight 113,000, Carter and Etzler, 1975) consists of four subunits with a possible stoichiometry of IA2IIA2. Other possible arrangements of the subunits are discussed.